1. Field of the Invention
Embodiments of the invention relate to isolated switching power supplies.
2. Description of the Related Art
FIG. 5 shows an example of isolated switching power supply having a main circuit 2 including a primary winding P1 of an isolation transformer T connected to an input power supply Vin and a switching element SW connected to the primary winding P1. The switching element SW is composed of an IGBT or a power MOSFET, for example. The switching element SW is ON/OFF driven by a control circuit 3 constructed as a power supply IC, for example, to generate an AC power in a secondary winding S and an auxiliary winding P2 of the isolation transformer T.
The AC power generated in the secondary winding S of the isolation transformer T is rectified through a diode D1 and smoothed through a capacitor C1, and then delivered to a load (not illustrated in the figure) as a specified DC output voltage Vout. The diode D1 and the capacitor C1 connected to the secondary winding S of the isolation transformer T compose an output circuit 4 of the isolated switching power supply 1. The AC power generated in the auxiliary winding P2 of the isolation transformer T is rectified through a diode D2 and smoothed through a capacitor C2, and then delivered to the control circuit 3. The diode D2 and the capacitor C2 connected to the auxiliary winding P2 of the isolation transformer T compose an internal power supply circuit 5 for generating a driving power supply voltage Vcc of the control circuit 3.
The output circuit 4 is provided with an output voltage detecting circuit 6 that detects the output voltage Vout dividing the output voltage. The output voltage detecting circuit 6 comprises for example, resistors R1 and R2 connected in series that divide the output voltage Vout and obtain an output monitoring voltage. The output monitoring voltage detected by the output voltage detecting circuit 6 is fed back to an FB terminal of the control circuit 3 through a photo-coupler PC.
The control circuit 3 controls the ON/OFF driving of the switching element SW as described previously corresponding to the fed back output monitoring voltage to stabilize the output voltage Vout at a constant value. The feedback control of the switching element SW can be carried out, for example, by changing the ON/OFF widths of the switching element SW or by changing the switching frequency of the switching element SW. The isolated switching power supply constructed as described above is called a power supply of a secondary side feedback type
The isolated switching power supply shown in FIG. 6, in contrast, obtains the output monitoring voltage from the driving power supply voltage Vcc generated by the internal power supply circuit 5 that is provided with an output voltage detecting circuit 6, in place of obtaining the output monitoring voltage in the output circuit 4 as in the isolated switching power supply of FIG. 5. The output monitoring voltage detected by the output voltage detecting circuit 6 in the isolated switching power supply of FIG. 6 varies corresponding to the ratio of winding numbers Ns:Naux, where Ns is the winding number of the secondary winding S and Naux is the winding number of the auxiliary winding P2. The isolated switching power supply constructed as described above is called a power supply of a primary side feedback type, and described in detail in U.S. Patent Application Publication No. 2012/0170326, for example. The isolated switching power supply of a primary side feedback type can feedback the output monitoring voltage to the control circuit 3 without using a photo-coupler PC, which reduces the number of components and is an advantage of that type of isolated switching power supply.
These types of isolated switching power supplies shown in FIGS. 5 and 6 set a low power standby mode in a light load period with a low output current in order to save power. The low power standby mode decreases the output voltage Vout when the output current from the output circuit 4 is small and the load (not shown in the figure) needs very little power, and reduces the power consumption in the isolated switching power supply.
The low power standby mode is generally set by giving an external control signal to change a voltage dividing ratio of the output power supply voltage Vout in the output voltage detecting circuit 6 in FIG. 5 or a voltage dividing ratio of the driving power supply voltage Vcc in FIG. 6. More specifically as shown in FIG. 5 and FIG. 6, an auxiliary resistor R3 is provided through a switch Sa in parallel to the resistor R1. The switch Sa is turned ON by the control signal to connect the auxiliary resistor R3 in parallel to the resistor R1 thereby changing the voltage dividing ratio of the output voltage detecting circuit 6. This change of the voltage dividing ratio changes the level of the output monitoring voltage fed back to the control circuit 3 to decrease the output voltage Vout. The switch Sa and the auxiliary resistor R3 compose an output voltage controlling means 7.
When the low power standby mode is set, the control circuit 3 generates an idle period, for example, in the ON/OFF driving of the switching element SW to restrict an average output power through the isolation transformer T, which is an effective output power. The intermittent operation control of the switching element SW as mentioned above by the control circuit 3 is generally called a burst operation mode.
When the low power standby mode is set in an isolated switching power supply of the primary side feedback type, a control signal has to be given to the isolated switching power supply from an external device in the load side. This process needs isolation of the control signal between the external device and the control circuit 3. The control signal isolation can be readily implemented by using a photo-coupler, for example. However, the newly required photo-coupler cancels the advantage of the isolated switching power supply of a primary side feedback type, which can reduce the number of components.